It is known in the art relating to the operation of weaving looms that such looms must be provided with components for conveying the weft yarns and components for moving the yarns forming the warp.
The components for conveying the weft yarns may be of different types, including those of the so-called gripper type and those of the air type, depending on whether the yarn is conveyed by mechanical components, known as grippers, or by directional jets of compressed air.
The components for moving the warp yarns are instead called weave machines and may be of the Jacquard or dobby type. These machines, particularly in the case of gripper looms, must be able to be operated in synchronism with the components conveying the weft yarns, so as to form the planned fabric design.
It is also known that, during weaving, breakage of the weft yarn may occur and in such cases, in order to avoid having a defect in the finished piece of cloth, it is necessary to interrupt weaving and, while keeping the components conveying the weft yarn at a standstill, to cause backward movement of the weave machine, undoing the fabric which has formed from the time the loom has stopped, in order to restore the continuity of the interrupted weft yarn.
While in the case of an air loom, the latter operation may be relatively simple, in the case of gripper looms it is necessary to separate the dobby from the loom and perform the backward movement thereof into the required position using an auxiliary low-speed drive.
In order to start again the weaving process it is therefore necessary to restore the coupled condition between loom and dobby, bringing them back into the relative angular position present at the time of their separation, so as to ensure the same synchronism between the relative movements occurring at the time of the interruption.
For this purpose, EP 0,322,928 discloses, for example, dual-clutch electromagnetic couplings which allow the loom to be coupled with the dobby via the first clutch, or the two of them to be separated so as to connect, via the second clutch, the dobby to an auxiliary drive by means of which an independent movement of the dobby is possible.
These dual-clutch couplings, however, have considerable dimensions and weight, particularly on account of the rotor element of the clutch integral with the shaft of the weaving loom, which element must interact with the respective electromagnet in order to produce the magnetic flux field designed to recall, in the axial direction, the clutch element which is coupled with a corresponding element of the dobby clutch.
Said rotor, although being axially fixed, requires large dimensions in order to avoid air-gap interference between rotor and electromagnet-carrying body fixed to the machine, with an increase in the costs of production and assembly of the coupling.